I'm in a similar situation, as I took 10 years off from riding, then came back to it in late December 2010.
We did only walk-trot for the first 2-3 months. I was/am out of shape, my legs were not strong and it took some time for my seat to come back to me. We cantered for a little bit in late February/early March. Then I took almost a month off because of work and scheduling problems. The mare I'd been cantering on had a *wonderful* slow, western type lope. The only problem was she'd throw her head a little in the beginning, and about 6-8 strides into it, would break into a trot. Every time. It's like just as I was beginning to get into the rhythm in 2-point and start settling into the saddle, she'd break.
Well, I came back to lessons in late April, did walk-trot for a while. My instructor would ask me if I wanted to canter, and I'd think about it, then talk myself out of it. Then two weeks ago, she gave me an ultimatum -- canter at the next lesson, or she would take my stirrups away.
(I'm 32, btw. I felt like *such* a little kid when she said it.)
So, I cantered in the next lesson.
Just prior we had done a nice lunge lesson, so I could solidify my seat a bit, and learn how to balance without reins. (Which is a WONDERFUL thing to do, btw.) Towards the end, while we were in the round pen, I had him pick up a canter. Which felt odd as hell, since we were moving in a circle. Then we moved to the regular ring and I cantered him 2x more. (In short bursts.)
The first time, about 4 strides in, his back foot slid out a bit. The second time, he picked up the wrong lead, which threw me off balance and it felt 'quicker' to me. Not once did I feel secure enough to come out of 2-point/half seat. It wasn't as smooth as I'd liked, but now I know when I go out for a lesson, if I don't canter when she asks, I know she's going to take away my stirrups. There's no buildup of 'Am I going to do it? Am I going to do it? No, I can't do it today.' There's no real anxiety to it. Just a sort of defeated acceptance. OK. We'll canter. /Eeore
Now, ultimatums probably aren't the best tactic for everyone, but it gave me enough of a reason to do it. Even though I'd already been doing 1' jumps at a trot and really and truly trust this horse and my trainer, I just couldn't push through with doing it when asked.